38 PRACTICAL BOTANY. 



strong sulphuric acid ; notice the swelling-up of the 

 thickened cell-walls. 



The swelling-up of starch-grains may be observed in 

 the same way. 



The amount of the swelling-up may be estimated by 

 using a micrometer-eye-piece. 



The thickened cell-walls of pith-cells of Clematis 

 Vitalba, or those of seeds (Lupine, Date) are also 

 suitable material for this purpose. 



II. Osmotic Properties. 



These can be most easily studied in cells which have 

 coloured cell-sap. 



Cut a rather thick section of a piece of fresh beet- 

 root, and mount in water ; 



Observe, the thin cell-wall ; 



the layer of protoplasm (primordial 



utricle) which lines the cell-wall ; 

 the red cell-sap filling the cavity of the 

 cell (vacuole). 



Note that the red sap does not escape from unin- 

 jured cells. 



Examine a similar section which has been dipped for 

 a moment into alcohol ; the red sap diffuses out of the 

 cells. 



Hence it is evident that the colouring-matter cannot 

 diffuse out of a living cell, but diffuses readily out of a 

 dead cell. 



Mount another section in water, and run some 10 per 

 cent. Na Cl solution under the cover-slip ; it will be 

 seen that the red sap collects as rounded deeply- 

 coloured bodies in the centre of the cells. This is 

 due to the contraction of the primordial utricle. 



